Water used in conventional water tanks for keeping fish are purified by being passed through a layer of gravel or a filter medium made of synthetic resin fiber placed on the bottom or top of the tank. The layer of gravel or filter medium has two functions; it intercepts organic matter such as fish excretion and feed residue, and the microorganisms growing on the gravel or filter medium decompose such organic matter. In the conventional method, water passes through gravel or filter medium simply by gravity, and because it is low pressure, the layer of gravel or filter medium is clogged in three or four weeks and the microorganisms within the layer lose their ability to purify water by decomposing organic matter. In consequence, water within the tank becomes cloudy and chlorophyceae grow on the side walls and bottom walls of the tank to such an extent that one cannot look through it. Such water will be a favorite place for pathogenic and other miscellaneous bacteria that attack the fish kept in the tank. Thus, with the conventional method it is difficult to keep the water in fish tanks clear over an extended period. What is more, some microorganisms are the cause of fish diseases. Therefore, frequent water replacement and cleaning of the inside of the tank as well as the gravel or filter medium layer are required. This means not a little time and labor is required. Hence, an effective method has long been desired for purifying water in tanks used for keeping fish in homes or aquariums.
It is known that such organic matter can be effectively decomposed and removed by a biological oxidation process that uses microorganisms. Typical examples of such biological oxidation treatment are (1) catalytic oxidation by trickling filtration that uses a film of microorganisms and which is suitable for the treatment of waste water having a relatively low concentration of contaminants and (2) suspension oxidation by activated sludge process suitable for the treatment of waste water having a high concentration of contaminants. When water contained in an aquarium is subjected to biological oxidation by passing it through a gravel bed of a synthetic fiber filter medium, the gravel bed or the filter medium is soon blocked, say, in a few weeks, and its refining capacity is reduced and the water becomes contaminated.
The conventional technique uses only method (1) and, as already mentioned, is affected by filter clogging within a relatively short time and fails to achieve the intended effect. In such conventional technique, chlorophyceae that grow on the tank side walls or bottom walls are removed by a suitable means for scraping the walls, or instead, an anti-algal paint is coated onto the walls of the tank or an algicide of low fish toxicity is supplied to the water. However, these methods have their own problems: with the first method complete cleaning is difficult to achieve; in the second method, a substance toxic to fish is released into water; in the third method, one algicide gives only a temporary effect, and another exhibits a lingering residual effect that is toxic to fish, or chlorophyceae that have died in large numbers will putrefy and decrease the oxygen dissolved in water and/or emit a malodor.